The present invention relates generally to a method for determining the neutron exposure and the constituent concentrations of an object such as a reactor pressure vessel. It is a radiometric technique based upon the fact that such neutron exposure induces radioactivity in the form of characteristic gamma-rays.
This disclosure relates specifically to a system originally developed for determination of reactor pressure vessel neutron exposure by a non-destructive technique. Such determinations are important in ascertaining the projected useful life of an active nuclear reactor. This non-destructive technique is an alternative to conventional techniques for directly counting activity in a sample area that is physically removed from the pressure vessel.
The present method of neutron dosimetry utilizes available continuous gamma-ray spectrometry techniques in a specialized physical application to achieve an effective non-destructive testing process for the nuclear industry A portable probe having a specially shielded detector is partially exposed to one or more surfaces through a collimator opening that is directed toward the surface area being tested. By measuring the resulting continuous spectrum of detected gamma-rays at different energy levels and recording one or more flux density peak values of the gamma-rays at energy levels characteristic of neutron exposure, one can mathematically derive a relationship between the flux density peak values and the spatial activity density. However, since this relationship will also include a second unknown--the neutron attenuation coefficient of the object being tested--it is necessary to either conduct separate tests to measure the neutron attenuation coefficient, or to make another set of measurements with a different collimator arrangement at a second solid angle and then mathematically solve the resulting relationships to determine the two unknown quantities expressed within them. Once the spatial activity density value of gamma-rays at the characteric energy levels is identified, one can deductively derive information concerning the spatial distribution of the neutron exposure by using known relationships between spatial activity density values and neutron exposure.
A general discussion of the Compton effect in lithium-drifted germanium detectors utilized in gamma spectroscopy can be found in U.S. Pat. No. 3,612,869 and in U.S. Pat. No. 3,527,944. A collimated radiation assembly to sequentially expose portions of an object to X-ray radiation is disclosed in U.S. Pat. No. 4,203,037. Other U.S. Patents of general background interest with regard to this invention are U.S. Pat. Nos. 3,483,376; 4,345,153; 2,998,550; 3,786,253; 3,043,955; 3,225,196; and 3,311,770.
It should also be appreciated that the embrittlement of reactor pressure vessel steels is a significant factor in determining the expected useful life of the pressure vessel. Because weldments are generally the weakest regions in such vessels, their anticipated life usually governs the productive life of the vessel. Since copper is a crucial variable contributing to radioactive induced embrittlement of steel bodies, the capability of nondestructively determining or measuring the copper concentration in the pressure vessel base metals, and more particularly in the weldments thereof, becomes highly desirable. The present invention, in addition to providing a method for non-destructively determining the neutron exposure of irradiated bodies, also can be utilized to measure the copper concentration, as well as other constituent concentrations, of the pressure vessel base metals and weldments.